This one's about lost treasure. Two boys, Larry and Josh, are the ones who find it. The circumstances are unusual. There's a tidal drop in the waters around the island were the boys live (Peolle Island, not a real place). When the sea rolls back, old wrecks are exposed. Just about everybody wants to find treasure, and when somebody does, there's bound to be someone who wants to take it away. So we have the dastardly Jimson brothers, who are willing to kill Larry and Josh if that's what it takes. The sea rolls back in, of course, as it always does, and what Larry and Josh are left with is . . . you'll have to read the book to find out.
Far be it from me to say anything (very) bad about a book by Mickey Spillane, but this one lacks the narrative drive of his novels for adults. It also has some clumsy writing and could've used a better editor. Still, it's fun for what it is, and it's worth a look if your curious about what Spillane could do outside his usual playground.